How to Resize Mac Partitions

Resizing a partition on your Mac may sound like a daunting task, and for some, it may sound like one that is better left to a technician at the Genius Bar. Messing around with something like this may seem scary but believe me, it’s really not that difficult to do.

The Disk Utility app included with the latest macOS will step you through this and will warn you if there will be any possible data loss, giving you the chance to cancel out of the process if you don’t feel comfortable. In most cases, it can be done without removing any data from your drive.

My name is Eric and I’ve been working with all types of computer systems over the past few decades. I enjoy learning how to do seemingly complicated things like resizing a disk partition and then passing the information on to others.

Administration tools for your computer have come a long way since the early days when I started working with computers. Doing a task like this required a lot of command-line typing but the new tools provided by Apple make processes like this quite easy and straightforward.

If you would like to see how this is done, stick around and we will step through it!

What You Will Need to Get Started

There aren’t many requirements needed to perform this activity. You really just need your Mac and a little time, but below are a few suggestions before you start.

Backup

Whenever performing a major operation like this to your disk drive it is highly suggested that you have a backup of your files and data. Use whatever tools work for you to back up your system. While this process should not remove anything from your drive, it pays to be safe just in case.

If you’re like me and save all your files and data to the cloud or external devices, you may not need to worry as much but just keep in mind, if something goes wrong you could lose everything currently on your drive. 

Having a backup of your files will allow you to restore everything if the worst should happen.

Power Adapter

I always recommend plugging in your power adapter when doing something like this. There’s nothing worse than your battery dying in the middle of modifying your hard drive. Plug into your AC power to ensure this doesn’t happen.

Confidence and Courage

Have the confidence and courage to perform this task. While it is true that things can go wrong, they most likely will not. Your worst-case scenario is that you have to completely wipe and reformat your hard drive, reinstall macOS and restore your data from backup

That sounds scary as well but resetting your Mac back to factory settings is a pretty simple process these days and as long as your important files are backed up or safe somewhere, you don’t have much to lose except for a little time.

How Will You Resize?

Before we get started, let’s first talk about what kind of resizing you are going to do because that will determine whether or not data will need to be removed from your drive. Do you need to increase the size or decrease it?

Increasing

Most people will want to increase the space. Of course, we all want more space! To do that you need to have space available on your hard drive to add to the partition that you wish to increase. If you already have unused space on another, the Disk Utility can increase it.


You will first need to reduce the size of the unused partition and then you can claim that space on your original one.

Decreasing

If you wish to decrease the space, such as in the case above where you want to use it on another partition, this is a little easier. The only caveat is that you need to have free space in order to decrease it. 

In other words, if that partition is already completely full of data, you won’t be able to reduce the size of it unless you remove some of the data and files first.

Resizing Your Partition: Step-by-Step Guide

Now that you have what you need and understand the difference between increasing and decreasing space, let’s get started. Since I have no extra partitions on my drive we will first decrease the size of my main partition, therefore creating a new one with unused space.

For this example, I will reduce the size of my 245 GB partition by 45 GB to make it around 200 GB and create a new 45 GB partition of unused space. 

Step 1: Open the Disk Utility app. You can simply go to LaunchPad and type Disk Utility in the search field at the top and it will find it for you.

Step 2: In the Disk Utility app, click on the Partition button at the top right side of the window.

This will open the Partition window and you will see a pie chart showing the space used by each of your partitions. As you can see mine has loads of free space so we can easily decrease the size without affecting any existing data.

Step 3: Click in the Size field and enter the new size that you wish it to be. For our example, I will change it to 200 GB and then click somewhere else in the window.

Notice how the pie chart has been updated to show a preview of the new partitions and space.

Step 4: Click on Apply and then confirm the action.

The confirmation pop-up tells me that the partitions will be changing and that none of them will be erased. If any data were going to be erased it would say so here and give you a chance to cancel out of this if you are uncomfortable with the action.

Step 5: Click on the Partition button in the pop-up window. You will then see another pop-up window warning you that the computer will be unavailable for w while during this process.

Step 6: Click on Continue to start the process. You will now see a status window showing the progress of the process which will take several minutes. During this time you will not be able to use the computer.

Step 7: When the process is complete you will see the results on the status screen. Click on the Done button.

The process will now be completed and if you go back to the Partition area in the Disk Utility you will see the new partition and new size of your main partition.

We can now go back and increase the size of our main partition by deleting the new 45 GB partition that we just created. Once it is deleted it will go back into our original partition.

From our example above we will increase the size of our main partition using the space from the new one we just created. The new empty partition is 45 GB, so we can delete that and our main partition will go back to its original size of 245 GB.

Follow these steps to delete the 45 GB partition and add it back to the original partition.

Step 1: In the Partition window, select the newly created 45 GB partition.

Step 2: Click on the button below the pie chart to remove the partition. You will now see a preview of how the partitions will look once the process is completed. You can see that the main partition will now be back to 245 GB.

Step 3: Click on the Apply button to start the process. You will see a message stating that the partition will be permanently deleted and anything on that partition will be deleted.

Step 4: Click on the Partition button to continue.

The resizing will begin and show a status window just like it did in the previous steps. Once the process has finished, click on the Done button and the space will be back with the original partition and it will show 245 GB again.

Final Words

As you can see the Disk Utility tool makes it easy to create new partitions, change their size and remove them. As long as you have plenty of free space to work with, you don’t really have to worry about losing data but it is still a good idea to create a backup first.

I hope this tutorial has helped you to learn how to change the size of Mac partitions. Please let us know if you have any questions or comments. We always appreciate the feedback.

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